Field of the Invention
The present invention relates to a motor driving apparatus.
Description of the Related Art
In recent years, increase in the operation speed of personal computers and workstations has led to rapid increase in the operation speeds of computation LSIs (large Scale Integrated Circuit) such as CPUs (Central Processing Unit), DSPs (Digital Signal Processor), etc. Such LSIs have a problem in that an increase in the operation speed, i.e., an increase in clock frequency involves an increase in heat generation. The heat generation of the LSI leads to thermal runaway of the LSI itself, or affects its peripheral circuits, which becomes a problem. Accordingly, such a situation requires a suitable thermal cooling operation for the LSI or the like, as a crucial technique.
In many cases, in order to cool such an LSI, an electronic device employs an air-cooling system using a cooling fan as a cooling method. In this cooling method, for example, a cooling fan is arranged such that it faces the surface of the LSI so as to blow cool air onto the surface of the LSI. In the cooling operation of such a cooling fan for cooling the LSI, the temperature in the vicinity of the LSI is monitored, and the rotation of the fan is adjusted based on the temperature thus monitored, so as to adjust the cooling level.
FIG. 1 is a circuit diagram showing a cooling apparatus including a fan motor driving IC (Integrated Circuit) investigated by the present inventors. It should be noted that any kind of configuration as shown in FIG. 1 cannot be recognized as a known technique.
A cooling apparatus 2r includes a fan motor 6 and a driving apparatus 9r that drives the fan motor 6. The driving apparatus 9r is configured including a driving IC 200r and its peripheral components. The components of the driving apparatus 9r are mounted on a common printed circuit board.
The fan motor 6 is configured as a brushless DC motor. A Hall sensor 8 is arranged in the vicinity of the fan motor 6 in order to detect the position of a rotor. The first pin and the sixteenth pin configured as a ground terminal (GND) are each grounded. The power supply voltage VDD is input to the third pin (VCC) of the driving IC 200r via a reverse-current blocking diode D1. The output of a driving stage 230 is connected to the fan motor 6 via the second pin (OUT2) and the fifteenth pin (OUT1). It should be noted that, in the present specification, each pin number is defined for convenience. That is to say, there is no relation between the pin number definition and the pin layout or the like.
A Hall bias circuit 204 generates a Hall bias voltage VHB, and supplies the Hall bias voltage VHB thus generated to the Hall sensor 8 via a Hall bias (HB) terminal configured as the tenth pin. Hall signals H+ and H− generated by the Hall sensor 8 are respectively input to Hall input terminals (H+ and H−) configured as the ninth pin and eleventh pin. A Hall comparator 202 compares the Hall signals H− and H+, generates a pulse signal S1 which indicates the position of the rotor, and outputs the pulse signal S1 thus generated to a control logic circuit 208. The control logic circuit 208 performs a commutation control operation in synchronization with the pulse signal S1.
A reference voltage source 214 generates a reference voltage VREF stabilized to a predetermined voltage level. The reference voltage VREF is output to an external circuit via a reference voltage terminal (REF) configured as the twelfth pin.
A capacitor C1 is connected as an external component to the oscillator terminal (OSC) configured as the sixth pin. An oscillator 220 charges and discharges the capacitor C1 so as to generate an oscillator voltage VOSC having a triangle waveform.
A minimum rotational speed setting terminal (MIN) configured as the fourth pin receives, as its input signal, a voltage VMIN which indicates the minimum rotational speed to be set for the fan motor 6. The voltage VMIN which is input to the MIN terminal, is generated by dividing the reference voltage VREF by means of resistors R11 and R12.
A PWM comparator 216 compares the voltage VMIN input to the MIN terminal with the oscillator voltage VOSC. An output S2 of the PWM comparator 216 has a duty ratio that corresponds to the voltage VMIN input to the MIN terminal.
A PWM comparator 218 compares a voltage VTH input to a rotational speed control terminal (TH) configured as the fifth pin with the oscillator voltage VOSC. An output S3 of the PWM comparator 218 has a duty ratio that corresponds to the voltage VTH at the TH terminal.
A PWM input terminal receives, as its input signal, an input PWM signal having a duty ratio (input duty ratio) that corresponds to a target rotational speed for the fan motor 6. The input PWM signal is inverted by an inverter 10. Subsequently, the input PWM signal thus inverted is smoothed by an RC filter 12, and is input to the TH terminal.
The control logic circuit 208 logically combines the output pulses S2 and S3 respectively output from the PWM comparators 216 and 218, so as to generate a pulse signal S4. The duty ratio of the pulse signal S4 is set to the larger of the output pulses S2 and S3 respectively output from the PWM comparators 216 and 218.
The driving stage 230 includes Hall amplifiers 232 and 234. The Hall amplifier 232 amplifies the difference between the Hall signals H+ and H− with a first polarity, and outputs the signal difference thus amplified via the OUT2 terminal. The Hall amplifier 234 amplifies the difference between the Hall signals H+ and H− with a second polarity, and outputs the signal difference thus amplified via the OUT15 terminal. The Hall amplifiers 232 and 234 each include a push-pull output stage. The respective output stages of the Hall amplifiers 232 and 234 switch on and off according to the pulse signal S4 received from the control logic circuit 208. The output voltages of the OUT1 terminal and the OUT2 terminal are alternately set to an active state according to the output S1 of the Hall comparator 202 (commutation control operation). In the active state, the corresponding output voltage has a waveform with an envelope obtained by amplifying the Hall signal. Furthermore, the output voltage is switched between an on state and a high-impedance state with a duty ratio that corresponds to the output pulse S3 (or S2) of the PWM comparator 218 (or 216).
A lock protection circuit 240 detects a motor lock state that can occur in the fan motor 6. A TSD circuit 242 detects an overheating state. A signal output circuit 244 generates an alert signal which indicates a malfunction, and outputs the alert signal via an alert terminal (AL) configured as the eighth pin. Furthermore, the signal output circuit 244 generates an FG (Frequency Generator) signal having a frequency that corresponds to the rotational speed of the fan motor 6, and outputs the FG signal via an FG terminal configured as the seventh pin.
FIG. 2 is an operational waveform diagram showing the operation of the driving IC 200r shown in FIG. 1. It should be noted that the vertical axis and the horizontal axis shown in the waveform diagrams and the time charts in the present specification are expanded or reduced as appropriate for ease of understanding. Also, each waveform shown in the drawing is simplified or exaggerated for emphasis for ease of understanding. FIG. 2 shows expanded waveforms in a sufficiently short time scale as compared with the periods of the Hall signals H+ and H−.
Accordingly, in the range shown in FIG. 2, the waveforms of the Hall signals H+ and H− each have a substantially constant voltage level. The output OUT1 has a duty ratio that corresponds to a comparison result obtained by comparing the oscillator voltage VOSC with a lower voltage from among VMIN and VTH. With such an arrangement, the torque (rotational speed) of the fan motor 6 is raised according to an increase in the duty ratio of the input PWM signal. Furthermore, such an arrangement allows the minimum torque, i.e., the minimum rotational speed, to be set according to the voltage VMIN applied to the MIN terminal.
The inventor has investigated the driving IC 200r shown in FIG. 1, and has come to recognize the following problems.
[Problem 1]
FIGS. 3A through 3C are diagrams showing, for the driving apparatus 9r shown in FIG. 1, the relation between the input duty ratio and the voltage VTH at the TH terminal, the relation between the input duty ratio and the output duty ratio of the output OUT1 (OUT2), and the relation between the input duty ratio and the rotational speed. As shown in FIG. 3A, the voltage VTH at the TH terminal is changed in a linear manner according to the input duty ratio of the input PWM signal. Thus, as shown in FIG. 3B, the duty ratios of the outputs OUT1 and OUT2 (output duty ratios) are changed in a linear manner according to the input duty ratio.
FIG. 3C shows the relation between the input duty ratio and the rotational speed of the fan motor 6. FIG. 3C shows an ideal characteristics curve (i) in an ideal case assuming that the fan motor 6 operates with no load and no power loss. In actuality, as shown in the actual characteristics curve (ii), an actual operation provides low performance as compared with the operation shown in the ideal characteristics curve (i) due to heat generation in the motor coil, friction loss in the bearings, windage loss accompanying the rotation of the rotor, and the effects of heat generation that occurs in various kinds of components of the motor. Such effects increase according to an increase in the rotational speed. With such an arrangement, there is an unavoidable problem in that, as the rotational speed becomes higher, the rotational speed is compressed as the input duty ratio becomes larger.
[Problem 2]
A related technique has been disclosed in Patent document (Japanese Patent Application Laid Open No. 2009-296839). An arrangement is described in this document in which a PWM signal is read out, compensation calculation is performed so as to provide a compensation signal, a compensation value is added or subtracted based on the compensation signal, and the rotational speed of a fan is controlled according to the compensated PWM signal.
In practical use, such a driving IC is combined with various kinds of fan motors. The rotational characteristics of the fan motor shown in FIG. 3C vary according to the kind of fan motor 6, the shape and size of the fan, and the heat-releasing performance of the fan motor 6 and the driving IC 200r. Accordingly, it would be useful to provide a technique for setting the optimum correction characteristics for every situation in which the driving IC 200r is employed.